Electrochemical investigation of plasmonic near-field and hot electron effects promoted by aluminium nanostructures

Electrochemical investigation of plasmonic near-field and hot electron effects promoted by aluminium nanostructures Introduction Recently, plasmonic nanostructures have been employed for harvesting solar energy for... [ view full abstract ]

Electrochemical investigation of plasmonic near-field and hot electron effects promoted by aluminium nanostructures

Introduction

Recently, plasmonic nanostructures have been employed for harvesting solar energy for photovoltaics and photocatalytic chemical reactions. There efficient use requires a close investigation of the involved plasmonic mechanisms. In the present work, plasmonic near-field and hot electron effects promoted by aluminium nanotriangles (AlNTs) were investigated in different configurations using electrochemical photocurrent measurements.

 Methods

 Three different configurations were developed and fabricated, using colloidal lithography, to study i) plasmonic hot electrons (ITO-AlNTs) ii) near-field coupling to a semiconductor (ITO-AlNTS-TiO₂) and iii) hot electron transfer to a semiconductor (ITO-TiO₂-Pt-AlNTs). A 40 nm Al and 100 nm TiO₂ films were deposited using electron beam evaporation whereas atomic layer deposition was used to deposit a uniform Pt layer. The optical response of the photoelectrodes were explored using UV-vis-NIR spectrometry and subsequently compared with numerical simulations. Electrochemical photocurrent measurements were performed in a three electrode photoelectrochemical cell using amperometric technique. A collimated narrow banded LED beam with its central wavelength at λ =365 nm was used to irradiate the photoelectrodes.

 Results and Discussion

 A SEM image of the AlNTs and their size distribution are shown in Fig. 1. The UV-Vis-NIR extinction spectra matches well with the numerical simulations showing a plasmonic peak around 365 nm (Fig. 2). Numerical simulations revealed the highest plasmonic electric field intensities at the edges of the AlNTs (inset, Fig. 2). Fig. 3 shows the photocurrent responses of the a) ITO-AlNTs, b) ITO-TiO₂, c) ITO-AlNTs-TiO₂ electrodes under illuminated conditions. The AlNTs on ITO does not show significant current due to the fast recombination of hot electrons. The bare TiO₂ photoelectrode shows a photocurrent of I = 33.2 μA corresponding to an external quantum efficiency (EQE) of 1.12 %. The photocurrent increases to 78 μA when AlNTs are brought close to the TiO₂ (ITO-AlNTs-TiO₂) that corresponds to an EQE, 2.7 %. Hence, plasmonic near-field coupling enhances the photocurrent ~2.4 fold. An additional 2 nm thin Pt layer forming a Schottky barrier prevents efficient recombination of the electron/hole pairs at the metal/semiconductor interface and favours hot electron injection into TiO₂.